It is widely known that friction materials show a load and frequency dependent stiffness. The load dependency can be explained with a geometrical nonlinear material structure, like the existence of porosities that are compressed by increasing the load. The frequency dependency is principally due to the viscoelastic characteristics of the polymer matrix that binds the different components together. Today there are commonly three different measurement systems used to investigate the material stiffness in compression: Ultrasound, which measures in a MHz-range, piezoelectric actuator to measure in a kHz-range, and hydropulser used for low frequency and quasi-static conditions .Comparing the results from different authors it can be observed, that the stiffness does not change dramatically in the frequency range of each system, but between systems. There is a great discrepancy in the observed results, and it is said, due to the material's frequency dependence. While ultrasound measurement systems usually provide the stiffest values, the hydropulser in the quasi-static range generates the softest values. The discrepancy between these measurements systems has been matter of discussion for quite some time in the industry.In this paper, a model approach using rheological elements is investigated to explain the differences in material stiffness observed during previous works from different authors. The investigation considers the basic mechanical effects present during the loading of a composite material that leads to different response behaviors depending on the measurement system. It will be shown once again, that friction materials are not only viscoelastic, being the main reason for the observed discrepancy using different measurement methodologies.